1. Field of the Invention
The present invention generally relates to a method for controlling redundancy and a transmission device, and particularly relates to a method for controlling redundancy and a transmission device for use in a ring network operating at a path rate lower than a line rate thereof.
2. Description of the Related Art
In recent years, there has been a growing demand for IP networks with greater reliability. RPR (Resilient Packet Ring) having a ring switching function for LAN (the term “LAN” as used herein includes WAN and MAN as well as LAN) is one promising technology to improve the reliability of the IP networks. In RPR networks, Ethernet™ frames are encapsulated in RPR frames. The RPR redundancy employs a steering method and a wrapping method, which are standardized in IEEE 802.17. Path failure information is transmitted between stations using control frames, thereby achieving reliability close to that of synchronous transmission systems such as SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy). It is understood that the term “SONET” is used hereinafter as short for “SONET/SDH”.
On the other hand, SONET communications networks provide SONET redundancy, in which a working (W) line is switched to a protection (P) line when a failure occurs on the working line, and the protection line is switched back to the working line when the failure is recovered from.
The existing infrastructure for the SONET communications networks is huge.
Currently, it is believed that using the existing SONET infrastructure is an effective way to readily and efficiently deploy the above-described RPR technology. However, if the RPR ring network is formed with use of the existing SONET infrastructure as a physical layer, both the SONET redundancy function and the RPR redundancy function are activated in the event of failure, resulting in a conflict.
In view of this problem, Patent Document 1 discloses a method for controlling the conflict between the SONET redundancy function and the RPR redundancy function by statically switching two modes with use of SONET overhead. In one mode, the RPR redundancy function is disabled while a BLSR (Bi-directional Line-Switched Ring) redundancy function, which is one of the SONET redundancy functions, is enabled. In the other mode, the RPR redundancy function is enabled while the BLSR redundancy function is disabled.    [Patent Document 1] Japanese Patent Laid Open Publication No. 2004-23480
FIGS. 1A-1C are block diagrams illustrating a related-art RPR over SONET network in which the existing SONET infrastructure is used as a physical layer. Referring to FIG. 1A, stations 1, 2, and 3 form a ring network through working (W) lines and protection (P) lines. The station 1 sends a data item α to the station 3, and a data item β to the station 2.
If a failure occurs on the working (W) line between the stations 1 and 2 as shown in FIG. 1B, the following operations are performed. With reference to FIGS. 1B and 2A, in the station 2, an optical interface section 4 detects a line error on the working (W) line and reports the line error to a CPU section 6. The CPU section 6 activates a SONET redundancy function such that connection to an RPR section 8 is switched by a switch section 7 from the optical interface section 4 for the working (W) line to an optical interface section 5 for the protection (P).
Once a SONET redundancy operation is completed by the SONET redundancy function in this way, a path of the RPR is restored. However, the RPR section 8 of the station 2 detects a path error before the completion of the SONET redundancy operation, so that an RPR redundancy function is activated although not needed.
When the RPR redundancy function is activated, a transmission route for the data item β is switched to a route of the station 1—the station 3—the station 2 as shown in FIG. 1C. Therefore, the available bandwidth between the station 1 and 3 decreases. Moreover, the unwanted activation of the RPR redundancy function causes a temporary signal interruption.
When the failure is recovered from, the following operations are performed. Referring to FIG. 2B, when a command is input or when a WTR (Wait To Restore) state is over, the CPU section 6 sends a signal such that the switch section 7 switches back connection from the optical interface section 5 for the protection (P) line to the optical interface section 4 for the working (W) line. Because a pointer is relocated during the switchback from the optical interface section 5 to the optical interface section 4, the RPR section 8 detects a path error due to the pointer relocation. As a result, the RPR redundancy function is activated although not needed.
As described above, although a path failure is recovered from by the SONET redundancy operation, the unwanted activation of the RPR redundancy function causes reduction of available bandwidth and temporary interruption of signals.
It may be a solution to this problem to increase the delay time of the activation of the RPR redundancy function with respect to the detection of the path failure using an RPR hold off timer; With this method, however, activation of the RPR redundancy function with the increased delay is applied even when RPR redundancy is really needed.
Turning back to the method disclosed in Patent Document 1, if both the working line and the protection line fail, i.e., if a double failure occurs in the mode where the RPR redundancy function is disabled, the RPR redundancy function cannot be activated.